Re: [PATCH v6 1/7] Documentation: DT: arm: add support for sockets defining package boundaries

[Sudeep Holla <sudeep.holla@arm.com>]

On Thu, May 30, 2019 at 08:56:03AM -0400, Andrew F. Davis wrote:
> On 5/30/19 7:51 AM, Morten Rasmussen wrote:
> > On Wed, May 29, 2019 at 07:39:17PM -0400, Andrew F. Davis wrote:
> > > On 5/29/19 5:13 PM, Atish Patra wrote:
> > > > From: Sudeep Holla <sudeep.holla@arm.com>
> > > >
> > > > The current ARM DT topology description provides the operating system
> > > > with a topological view of the system that is based on leaf nodes
> > > > representing either cores or threads (in an SMT system) and a
> > > > hierarchical set of cluster nodes that creates a hierarchical topology
> > > > view of how those cores and threads are grouped.
> > > >
> > > > However this hierarchical representation of clusters does not allow to
> > > > describe what topology level actually represents the physical package or
> > > > the socket boundary, which is a key piece of information to be used by
> > > > an operating system to optimize resource allocation and scheduling.
> > > >
> > >
> > > Are physical package descriptions really needed? What does "socket" imply
> > > that a higher layer "cluster" node grouping does not? It doesn't imply a
> > > different NUMA distance and the definition of "socket" is already not well
> > > defined, is a dual chiplet processor not just a fancy dual "socket" or are
> > > dual "sockets" on a server board "slotket" card, will we need new names for
> > > those too..
> >
> > Socket (or package) just implies what you suggest, a grouping of CPUs
> > based on the physical socket (or package). Some resources might be
> > associated with packages and more importantly socket information is
> > exposed to user-space. At the moment clusters are being exposed to
> > user-space as sockets which is less than ideal for some topologies.
> >
>
> I see the benefit of reporting the physical layout and packaging information
> to user-space for tracking reasons, but from software perspective this
> doesn't matter, and the resource partitioning should be described elsewhere
> (NUMA nodes being the go to example).
>
> > At the moment user-space is only told about hw threads, cores, and
> > sockets. In the very near future it is going to be told about dies too
> > (look for Len Brown's multi-die patch set).
> >
>
> Seems my hypothetical case is already in the works :(
>
> > I don't see how we can provide correct information to user-space based
> > on the current information in DT. I'm not convinced it was a good idea
> > to expose this information to user-space to begin with but that is
> > another discussion.
> >
>
> Fair enough, it's a little late now to un-expose this info to userspace so
> we should at least present it correctly. My worry was this getting out of
> hand with layering, for instance what happens when we need to add die nodes
> in-between cluster and socket?
>

We may have to, if there's a similar requirement on ARM64 as the one
addressed by Len Brown's multi-die patch set. But for now, no one has
asked for it.

--
Regards,
Sudeep